Interesting question, which I can answer very directly:Is that due to the amount you have seen, or that as you get older you find it less titillating due to more real life experiences? Perhaps a combination of both?
I'm 27 years of age and don't have practical experience. I've never even seen a real naked woman.
So the desensibilization I'm experiencing is purely related to images of persons.
I imagine that seeing a woman that is near and dear to your heart w/o clothes cannot really be compared to a depiction of a "photoshopped-perfect" random stranger.